Research on piston offset suppression and structure optimization for free piston linear compressors in pulse tube cryocoolers

As the core driving component of pulse tube cryocoolers (PTCs), free piston linear compressors are susceptible to piston offset in actual operation, affecting the stroke and performance. This paper employed a magnetic resonance linear compressor (MRLC) to drive PTC, utilizing the magnetic spring effect of magnetic resonance linear motor (MRLM) to suppress piston offset. Through a combination of experimental and finite element analysis (FEA), the paper systematically elucidated the piston offset characteristics of the linear compressor and optimized the MRLM structure. Research shows that piston offset is mainly driven by the mean pressure difference (MPD) between the compression pressure and back pressure. The MRLM exhibits a preload force of 54.1 N at the origin, and the magnetic spring stiffness has a non-symmetrical distribution with low stiffness on the left and high stiffness on the right. The magnetic spring effect in the MRLM, operating similarly to an eccentric spring, suppresses piston offset by utilizing the magnetic spring force at the origin (MSFO) and the asymmetry in magnetic spring stiffness on either side of the origin. The simulation demonstrated that the MRLC can maintain a negative offset of -0.75 mm under high charge pressure of 3.6 MPa, which is significantly lower than that of the Redlich-type linear compressor. Furthermore, by adjusting the dimensions of the linear motor, the MSFO can be regulated to reduce piston offset. This approach provides an alternative design strategy for suppressing piston offset in free piston linear compressors.